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SEO Ultimate is my favorite by far after playing with all the others.  Just make sure to install Google XML sitemaps as well since it isn't built in.

I see this quite a bit on my less content filled wordpress blogs.

If you have categories A and B and one post that fall under both categories crawlers see it like this.

Category A: Post 1

Category B: Post 1

In addition to the permalink for the post.  You can get the same problem with tags as well.

So there are multiple ways to see the same posts, which means duplicate posting.  Once more content gets added the category pages will look different so that tends to sort itself out over time.  Just explaining where it comes from.

I personally like SEO Ultimate but it doesn't quite solve this problem as I still see it on my sites at the beginning.  An easy fix would be to change your robots.txt to nofollow all category/tags.

Raven Tools just came out with a Schema plugin for Wordpress (named Schema-Creater) that can be found in the official plugin directory.

http://wordpress.org/extend/plugins/schema-creator/

I haven't used it on any of my sites but read reviews and tutorial and it looks pretty slick.  Gives you schema tags directly in the text editor.

Also the ultimate guide on schema is at:

http://schema.org

Raven Tools just came out with a Schema plugin for Wordpress (named Schema-Creater) that can be found in the official plugin directory.

http://wordpress.org/extend/plugins/schema-creator/

I haven't used it on any of my sites but read reviews and tutorial and it looks pretty slick.  Gives you schema tags directly in the text editor.

Also the ultimate guide on schema is at:

http://schema.org